The major difference is the points of entry and exit. If you enter Cambodia and exit from there through border check points eligible for e-visa (The list of these border crossings is here:http://www.visacenter.ca/Cambodia-E-visa-border-entry-points.pdf) , then you may apply for Cambodia electronic visa by filling out on-line application form: http://cambodia.visacenter.ca/eform

If you apply for e-visa there is no need to visit our office and surrender your passport for the processing.